"338 



We have here in miniature the outlines of a process 

 which, on a grander scale, Smyth < 25 ) has suggested for the 

 differentiation of alkaline from subalkaline magmas. 



Review of differentiation. — The predominant potassic- 

 aplites with the minor sodic-pegmatites, plus the volatile 

 mineralizers, represent, approximately, the residual magma 

 derived from the fractional crystallization of the granitic 

 magma. 



This residual magma under favourable conditions under- 

 went further differentiation, yielding predominant potassic- 

 aplites as the fractionally crystallized portion and the minor 

 sodic-pegmatites as the residual liquid highly enriched in 

 mineralizers, now represented by the presence of the accessory 

 minerals. 



In the main, the mechanism of differentiation appears to 

 have been one of straining off of a residual liquid from a 

 crystalline mass. 



It may well be that the extent of differentiation in such 

 cases is dependent on the magma chamber remaining undis- 

 turbed by external agencies for sufficiently long periods to 

 allow of delicate adjustment of equilibrium in the presence of 

 volatile mineralizers. 



With the crystallization of the albite-pegmatites differ- 

 entiation appears to have closed. At a late stage in the 

 consolidation of the microcline-aplite, the pneumatolytic action 

 of mineralizers is represented by the quartz-tourmaline 

 pneumatoliths, and at a still later stage greisenization and 

 kaolinization were developed. 



Correlation with other Australian Albitites. — These 

 albite-pegmatites, or albititesj are to be correlated with the 

 previously-described albitites from Pilbarra region, Western 

 Australia, and the albitites from Eyre Peninsula. In all 

 three cases their association appears to be with granitic rocks. 

 In the case of the Pilbarra rock, the pegmatite is tin bearing. 

 The Eyre Peninsula albitites are remarkable for the association 

 in one case of wernerite in long prismatic crystals. 



VII. General Discussion. 



Form of the Intrusion. — It has been noted in the Intro- 

 duction that the granite is distinctly transgressive to the 

 surrounding schists and quartzites. The seaward extension of 

 the granite is not known. Some evidence of the underground 

 extension of the granite, in an horizontal direction, is afforded 



(25) C. H. Smyth, jr.: Amer. Jour. Sci., 1913, 36, p. 42. 



